1. Technical Field
The present invention relates to gas generating material for a vehicle occupant restraint such as an airbag, and to a method for making the gas generating material. The present invention relates particularly to a booster coating for gas generating grains.
2. Description of the Prior Art
A known gas generating grain for a vehicle occupant restraint comprises an azide, such as sodium azide and a metal oxidant, such as iron oxide. The composition may also contain a forming aid, such as bentonite, strengthening fibers such as graphite fibers, and an inorganic source of oxygen, such as sodium nitrate. The ingredients of the composition are proportioned to obtain a desired burn rate, rapid ignition, and stability against inadvertent ignition.
It is also known to provide a gas generating grain with a booster coating which enhances ignition U.S. Pat. No. 4,806,180, assigned to the assignee of the present application, discloses a booster coating comprising 30-50 percent by weight of a metal azide, 40-60 percent by weight of an inorganic oxidizer, 5-15 percent by weight of boron, and 1-15 percent by weight of an alkali metal silicate. Sodium nitrate is disclosed as one suitable inorganic oxidizer. The boron produces heat to assist in igniting the grain to which the coating is applied. A preferred method of coating the grains involves first preparing a liquid coating mix in an appropriate container with a suitable solvent such as acetone or methyl alcohol. Water can also be used as the solvent. The grains are then placed in a steel mesh basket. The grains in the basket are immersed in the coating mix and then removed from the coating mix and dried.
A coating composition has been proposed which is applied to the grain as a paste. The coating includes sodium nitrate and sodium azide. The sodium nitrate is first pulverized in a micro-pulverizer and then blended with sodium azide and a binder. Both the sodium azide and the sodium nitrate before blending are screened through a 100 mesh screen. Alcohol is added to form a paste. The gas generating grains are coated with the alcohol paste. The use of alcohol, instead of water, as a solvent minimizes dissolution of the grain which is coated. A small amount of water is introduced as steam into the coating vessel. About 10 milliliters of water per fifty pounds of coating material is introduced into the coating vessel. This provides improved bonding of the coating to the grains. Following coating, the grains are placed in a 90.degree. C. (194.degree. F.) oven for overnight drying.
U.S. Pat. Nos. 4,696,705 and 4,698,107, assigned to assignee of the present application, disclose a coating composition for a nitrogen gas generating grain for a vehicle occupant restraint. The coating composition contains 10-15 percent by weight of a fluoroelastomer binder. The composition also contains 20-50 percent by weight of alkali metal azide, 25-35 percent by weight of inorganic oxidizer, 15-25 percent by weight of magnesium, and 1-3 percent by weight of fumed metal oxide. The ingredients are mixed with a suitable solvent and applied to the grain. The fumed metal oxide functions in the coating mix as a suspension agent and keeps the ingredients of the coating composition suspended in the mix so that a uniform coating is applied to the grain.
Coating compositions which are dissolved in an organic solvent for application to a gas generating grain are disclosed in U.S. Pat. Nos. 4,244,758 and 4,246,051.
A problem with an organic solvent-based coating, such as an acetone-based coating, is that vapors from the solvent of the coating create a fire hazard and/or may be toxic.